Common name: St Peter's Wort, Four Petal St. John's Wort
Plant Link: http://plantsdatabase.com/go/54216/
I think this picture is of Hypericum myrtifolium, which differs from H. tetrapetalum in having five petals instead of four. It also prefers wetter habitats (wet flatwoods, wet prairies, edges of depression marshes) than does H. tetrapetalum (scrubby to mesic flatwoods) and is more likely to do well in your pond edges. The two are the only species of Hypericum in Florida with leaves that are broadest and the base and clasping the stem.